1. Field of the Invention
This invention relates to a thermocompression bonding tool used for mounting a semiconductor device or element such as IC, LSI, etc. on a substrate plate.
2. Description of the Prior Art
Lately, various electronic devices using semiconductor elements have been developed and technical progress in this field has become remarkable. In order to draw out the electric properties of semiconductor elements and allow the properties to sufficiently be shown, it is required to bond electrodes formed on the semiconductor devices with leads of a package (inner lead bonding) and bond the leads with outer terminals of a printed circuit substrate (outer lead bonding).
Bonding of the electrodes of semiconductor devices with the leads of a package has hitherto been carried out by a method comprising bonding metallic fine wires (bonding wires) of gold or copper one by one by a tool called capillary, namely, by wire bonding. A mounting method by a wireless bonding technique, using no bonding wire, has lately been watched with keen interest, instead of this wire bonding technique, by excellent features, e.g. high mounting efficiency, large degree of freedom in package designing, etc.
TAB (Tape Automated Bonding) system in which all electrodes of a semiconductor device are in a lump thermocompression-bonded to a film carrier (a printed pattern is formed on a laminated tape of a Cu foil and resin, followed by plating with Sn or Au) has been put to practical use, for example, in mounting ASIC (Application Specific Integerated Circuit) or a liquid crystal display driver. Mounting of TAB system has often been used not only in the inner lead bonding but also in the outer lead bonding. In the outer lead bonding by TAB system, the outer lead of the film carrier is bonded with the printed circuit substrate or lead frame as an outer terminal. In the case of bonding with the printed circuit substrate, a lead of Cu plated with, for example, Sn is bonded with a substrate electrode of, for example, Au using a solder. In the case of bonding with the lead frame, Au on the film carrier tape is bonded with Ag on the lead frame by thermocompression.
For the thermocompression in the mounting by TAB system, there are used tools called bonding tools which can be classified into two kinds of a constant heating system and pulse heating system and can be used properly depending upon the properties of materials to be bonded, etc. Herein, the constant heating system is a system in which a tool is constantly heated directly or indirectly, pressed against a workpiece to be bonded and subjected to heating and melting bonding or thermocompression bonding. The pulse heating system is a system in which a tool is pressed against a workpiece under directly or indirectly heated state for a constant time, cooled to a predetermined temperature, followed by release of the compression against the workpiece, and then subjected to heating, melting and bonding or thermocompression. Namely, this pulse heating system is a system for carrying out heating and cooling of a tool with a constant cycle.
Furthermore, tools having markedly improved properties have lately been developed by the use of diamond as a tool end material. In particular, a tool using a tool end material comprising polycrystalline diamond coated on a specified substrate by a gaseous phase synthesis method (Japanese Patent Laid-Open Publication No. 224349/1990 and Japanese Patent Application No. 62905/1994) has widely been used because of the excellent heat resistance, wear resistance, etc. of the tool.
The constant heating tool is, for example, a tool having a shape as shown in FIG. 1, which comprises a tool shank 1 with a penetrating hole 2 for mounting a heater (not shown) and a tool end material 3 to be always maintained at a thermocompression temperature of 500 to 600.degree. C. by passing electric current through the heater to heat it. This tool is mainly used for bonding other bonding materials than solders.
On the other hand, the pulse heating tool is, for example, a tool 4 having a shape as shown in FIG. 2 or FIG. 3 and the tool of FIG. 2 is the most commonly used pulse heating tool made of a metal or alloy. This tool is mechanically fixed to a bonding machine via a hole 6 for fitting, and after pressing the tool end surface 5 to a workpiece to be bonded, the tool body 4 itself having electrical conductivity is self-heated by passing electric current with a pulse of several second unit corresponding to the cycle of bonding. The tool of FIG. 3 is designed in such a manner that a tool material 3 is bonded to the end of a shank 7 via a bonding metal 8 in the similar manner to the constant heating tool of FIG. 1. This tool aims at improving the wear resistance, heat resistance, etc. of the tool of FIG. 2 by the use of another material than the metals or alloys used in the tool body of FIG. 2. These tools are often used in the case of mainly using a solder for a workpiece to be bonded.
The pulse heating tool is preferably used for soldering, because in the case of a tool of the constant heating system, there arises a problem that the tool is still heated after bonding and the melted solder, adhered to the end surface of the tool, is drawn up with the tool, resulting in contact of bonded parts with each other, while in the case of the pulse heating system, the tool is withdrawn after cooled to a temperature of lower than the melting point of the solder and thus, such a problem does not arise to result in good bonding.
Even when a solder is not used, in many times, the pulse heating tool is more preferably used, since the inflow of heat into a bonding machine is less and the thermal breakage of the machine is hard to occur, as compared with the constant heating tool.
In TAB mounting of ASIC, TCP (Tape Carrier Package) mounted by semiconductor elements by the above described inner lead bonding is bonded with a lead frame using a bonding tool having a different shape. In this case, mounting is carried out by a system (constant heating system) comprising constant heating directly or indirectly a tool having an end shape of a hollow square frustum with an end surface of a rectangular frame to be a pressing surface as shown in FIG. 10 and pressing it to a workpiece to be bonded, made of Au or Ag, thus effecting the thermocompresion thereof.
Such a tool as having the same shape can be applied to a use of mounting a lead of an integrated circuit on a bonding part of a printed circuit substrate precoated with a solder by soldering. In the case of mounting by soldering, the constant heating system results in a problem that since the pressing of the tool is released and the tool is drawn up while a melted solder is adhered to the pressing surface of the tool, adjacent bonding parts are brought into contact with each other. This is not preferable. In this case, therefore, another system (pulse heating system) is employed comprising pressing a tool to a workpiece for a constant time while directly or indirectly heating the tool, then cooling to a constant temperature, releasing the pressing to the workpiece and heating, melting and bonding.
Among mounting techniques by the wireless bonding, in particular, mounting by the flip chip system is capable of corresponding to pitch-narrowing of substrate leads because of directly bonding with the substrate utilizing bumps formed on LSI and has been developed as a mounting method which can be expected to improve the mounting efficiency. In fact, this mounting method has been employed, for example, for mounting MPU of work stations or personal computers, mounting of driver LSI on glass substrates in the production of liquid crystal pannels, etc.
In the mounting of MPU, there is used a mounting method comprising using a tool directly or indirectly heated at a temperature range of 200 to 400.degree. C. to melt and bond a bump consisting of a solder, adsorbing in vacuum and transporting LSI, then pressing it for a constant time and cooling to 200.degree. C. or lower, after which the pressing of the tool is released.
Since in a process for the production of a liquid crystal panel, driver LSI is mounted on a glass substrate via a thermosetting ACF (anisotropic conductive film), in addition to the above described method, there is used a mounting method comprising adsorbing in vacuum and transporting LSI and then pressing it for a constant time under such a sate that the tool is directly or indirectly heated at a temperature range of 200 to 400.degree. C.
As to the properties of such a bonding tool, excellent heat resistance and wear resistance are required, since any one of the tools is constantly or intermittently maintained at a high temperature and a concentrated load is repeatedly applied to a thermocompression bonding part on the tool surface. From this point of view, a hard material consisting predominantly of diamond has been used as a tool material of bonding tool.
Of the tool materials having been used at the present time, single crystal diamond is most excellent as a material property, so this material corresponding to a size of 10 to 15 mm square as a shape of standard tool end under the existing circumstances in the constant heating tool as shown in FIG. 1 is very expensive and application thereof is limited to a small size tool.
On the other hand, sintered diamond can be obtained with a relatively large size, but has a problem, in particular, when using an iron group metal as a binder material as disclosed in Japanese Patent Publication No. 12126/1977, that the heat resistance is insufficient to result in a shortened service life. When the sintered diamond is used as a tool material, as disclosed in Japanese Patent Laid-Open Publication No. 33865/1986, this sintered body has a problem, due to use of Si and/or SiC as a binder for the purpose of improving the heat resistance, that bonding of diamond grains with each other is too weak to maintain a practical wear resistance.
In contrast, ceramics such as sintered bodies consisting predominantly of Si, Si.sub.3 N.sub.4, SiC or AIN, coated with polycrystalline diamond by a gaseous phase synthesis method, as disclosed in Japanese Patent Laid-Open Publication No. 224349/1990, exhibit excellent properties comparable to single crystal diamond and can be produced with a low cost, so that they have lately been applied to many uses.
From this point of view, it has been proposed to use a hard material consisting predominantly of diamond, instead of metals such as Mo, W, etc. as in the prior art, for the end material of a tool for the outer lead bonding. The inventors propose, as a first embodiment of the present invention, to use cemented carbides coated with polycrystalline diamond by a gaseous phase synthesis method as a tool end material of a tool capable of exhibiting excellent properties for uses of not only the inner lead bonding but also the outer lead bonding. FIG. 4 is a schematic view of one example of the tool structure of the present invention, comprising a diamond-coated substrate 3, tool end surface (polycrystalline diamond) 5, machine fitting parts 6, shank 7 and brazing material 8.
As described above, the tool described in Japanese Patent Laid-Open Publication No. 224349/1990 having various excellent features has broadly been used, but has some problems to be further solved. Firstly, in this tool, ceramics are used as a substrate to be coated with diamond and accordingly, a problem on strength often takes place depending on the using conditions. That is, the problem is lowering of the durability of the ceramics when a bonding load is large or the heating temperature of the tool is high.
Lately, the shape of a semiconductor device itself tends to be large-sized or long-sized with the increase of functions or integrations of the semiconductor device and for the purpose of improving the efficiency, it is begun to employ a system for mounting in lump a plurality of semiconductor devices. Correspndingly to this tendency, it is required for bonding tools to render the shapes thereof large-sized or long-sized. However, in the tool of such a shape, a lowering tendency of liability as to the strength of a ceramic substrate by a larger volume effect is undeniable, as compared with that of a small-sized shape.
Furthermore, another problem than the strength relates to a heat response when this material is used for a pulse heating tool. As apparent from the structure shown in FIG. 3, a pulsating instantaneous heat generation in a shank is propagated through the ceramic substrate and reaches the surface of polycrystalline diamond. Accordingly, the heat response of the tool, determining a mounting cycle, largely depends on the thermal conductivity of the substrate. In the case of the substrate disclosed in Japanese Patent Laid-Open Publication No. 224349/1990, in fact, if its material does not have highly thermal conductivity, the thermal conductivity of the tool is not sufficient and the mounting cycle is thus lengthened by at least two times as long as tools of metals or alloys, as shown in FIG. 2. This is a problem.
Therefore, it is considered most suitable to use a high strength and high thermal conductivity material as a substrate for an ideal TAB tool, which is coated with polycrystalline diamond by a gaseous phase synthesis method. From this point of view, cemented carbides are considered suitable as a substrate for a TAB tool.
The coating technique of polycrystalline diamond onto a cemented carbide substrate has actively been developed for the purpose of mainly aiming at applying to cutting tools and as to the bonding strength of a diamond film having hitherto been considered to be a problem, various improving methods have been proposed. In particular, a surface modifying method comprising subjecting cemented carbides to a heat treatment under special conditions, as disclosed in Japanese Patent Laid-Open Publication No. 330959/1993, is effective for improving the bonding strength. According to this method, the bonding strength between a cemented carbide substrate and diamond coating layer is improved by subjecting a WC-based cemented carbide having a composition comprising, as a binder phase component, 0.5 to 30% by weight of Co and, as a hard dispersed phase forming component, (a) WC, (b) Group IVa, Va and VIa metals of Periodic Table except W or solid solutions thereof with at least one of carbides, nitrides, carbonitrides, oxides, borides, borocarbides, boronitrides and borocarbonitrides thereof and (c) WC and/or (d) WC and Group IVa, Va and VIa metals of Periodic Table except W or solid solutions thereof with at least one of carbides, nitrides, carbonitrides, oxides, borides, borocarbides, boronitrides and borocarbonitrides thereof and unavoidable impurities to a heat treatment, thus modifying the surface thereof and then coating it with polycrystalline diamond by a gaseous phase synthesis method. In this publication, it is further described that when at least one of carbides, nitrides and carbonitrides of at least one of Group IVa, Va and VIa metals of Periodic Table except W is further contained as the hard phase, the high temperature hardness of the substrate can be increased by the presence of these carbides, nitrides and carbonitrides, preferably in a proportion of 0.2 to 40 weight %.
However, the materials described herein include a broad range of compositions varying in property and it has not been made to study which composition or which property is suitable as a material for bonding tools, of these materials.
When using a tool having the structure shown in FIG. 4 for soldering for a long time, there arises a problem that the polycrystalline diamond of the tool end part 5 is stripped from the brazed part to shorten the service life of the tool, in spite of that the polycrystalline diamond itself of the tool end part 5 is not so damaged. The cause of this problem consists in that the solder is melted and evaporated during bonding, adheres to the soldered part and diffused through the solder to change the composition of the solder and to form a brittle intermetallic compound, thus resulting in lowering of the bonding strength by the solder.
Furthermore, in order to improve the mounting efficiency by carrying out at once mounting of a plurality of electronic parts, a tool of several tens mm in length is not sufficient and accordingly, a long-sized tool has lately been desired. For example, in the mounting of a liquid crystal driver, a tool with a length of about at most 400 mm has been required. Even in the case of such a long-sized tool, it has been required for realizing a uniformly bonded state that the flatness of the pressing surface of a tool is at most 3 .mu.m and the maximum temperature gradient is at most 10.degree. C.
As the above described tool, for example, a tool of the pulse heating system using molybdenum or tungsten as the tool material is disclosed in Japanese Utility Model Publication No. 30142/1990. In the tool using such a metal, however, a part in contact with the bonding material is gradually subject to damage by the repeated heating and pressing, thus resulting in a problem that it is difficult to maintain a uniformly bonded state for a long period of time. In such a metallic tool, however, a cleaning working to remove periodically the solder or oxide adhered to the pressing surface is required, during which damage by a cleaning grindstone changes the flatness of the tool pressing surface and unfavorably affects the bonded state. This is a large problem.
In order to solve these problems, it has been considered effective to use a hard material consisting predominantly of diamond or cBN (cubic boron nitride) excellent in heat resistance and wear resistance as a tool end material. However, single crystal diamond is has poor pratcial utility because of being limited in size. In the case of a diamond or cBN sintered body, moreover, there is a problem that it is difficult to work it into an end shape as shown in FIG. 10 and to maintain the flatness of the end surface at a high temperature within a precision range required for a long time since the property of the sintered body is affected by a metallic or non-metallic binder.
In any mounting method, the bonding tool must have an excellent heat resistance as its property, since it is constantly or intermittently allowed to be present under high temperature state. That is, it is required of the tool to directly press LSI without breakage of LSI and to maintain the surface roughness and flatness of the tool end surface under good state without thermal damage for a long time. However, the use of a metallic tool of an Invar alloy or Mo, etc. having hitherto been used up to the present time results in a problem that the property is gradually deteriorated.
Furthermore, as referred to above, the tool surface should periodically be cleaned since the heated and sublimated solder or resin is solidified and adhered thereto. The cleaning is generally carried out by mechanically removing the adhered material, but during the same time, the tool of the prior art meet with a problem that the end surface is scraped to change the shape and a good mounting operation cannot be continued.